The invention relates to sending and receiving, and in particular to estimating transmission channels in a MIMO (Multiple Input Multiple Output) or MISO (Multiple Input Single Output) multi-antenna system using multiple carriers and equalization in the frequency domain. In a system with multiple send or receive antennas, there are as many transmission channels as there are sending antenna+receive antenna pairs. Channel estimation is the process of estimating the impulse response of each channel. The invention applies to multi-antenna multi-carrier systems using at least two send antennas.
These systems employ a frame of particular length having content at the input of the sender device that includes payload data symbols, i.e. symbols that code the information of an input signal, and at the output of the same device the frame is distributed in time and in frequency when sent on multiple carriers. A time-frequency frame then determines the temporal location of payload data symbols and pilot symbols, i.e., reference symbols inserted into the time-frequency frame on transmission, on the various carriers. Furthermore, the presence of multiple send antennas enables the introduction of spatial diversity by multiplexing the payload data between the antennas. In the remainder of this document, the term data refers to payload data.
The invention can be applied to uplink communication (from a terminal to a base station) and to downlink communication (from a base station to a terminal).
One example of application of the invention is the field of fixed or mobile radio communication, especially fourth generation and later systems typically referred to a B3G (Beyond 3rd Generation) systems. These systems include MC-CDMA (Multi-Carrier Coded Division Multiple Access) downlink or uplink systems and downlink or uplink OFDMA (Orthogonal Frequency Division Multiple Access) systems, using a MIMO (Multiple Input Multiple Output) transmission scheme, in which data to be transmitted is divided into time-frequency frames including pilot symbols and possibly null carriers. The invention applies in particular to any type of system using OFDM modulation, for example of OFDMA, LP-OFDM type, or to systems of IFDMA type.
Standard transmission methods include a modulation step. If differential modulation is not used (non-coherent system), it is essential for the receiver to estimate the propagation channel (coherent system) in order to be able to equalize the received signal and detect the bits sent. Differential modulation applied to multi-antenna systems is not at present considered to be a promising option for high-bit-rate communication systems. It doubles the noise level, which degrades performance by around 3 dB.
Thus, channel estimation is particularly important in multi-antenna systems because their performance is directly linked to channel estimation in the receiver. The various channels linking each send antenna to each receive antenna must also be estimated independently of each other. The performance of multi-antenna systems is further constrained by the presence of pilot symbols that lead to a loss of spectral efficiency.
A number of techniques for estimating transmission channels in a multi-antenna system comprising multiple send antennas are known in the art. They include techniques based on processing pilot symbols. These pilot symbols are known to the receiver and enable it to estimate the transmission channels corresponding to each send antenna. In theory, the capacity of MIMO systems increases in linear relation to the minimum number of send and receive antennas. In practice, because of the necessary presence of pilot symbols in the frame, the usable spectral efficiency is inversely proportional to the number of antennas.
Various techniques exist for inserting pilot symbols into the time-frequency frame sent by an antenna. All the pilot symbols for the same time-frequency frame form a training sequence.
A first technique, illustrated by FIG. 1a, described in the paper by Y. Teng, K. Mori and H. Kobayashi “Performance of DCT Interpolation-based Channel Estimation Method of MIMO-OFDM Systems”, ISCIT, October 2004, inserts an OFDM pilot symbol successively in time at each send antenna and sets to zero the OFDM symbols concomitant in time to a pilot OFDM symbol for all the other send antennas. A pilot OFDM symbol is an OFDM symbol containing pilot symbols. In this particular type of frame, a pilot OFDM symbol comprises only pilot symbols. FIG. 1a illustrates this technique for constructing type 1 frames for two send antennas TX1 and TX2. In the first OFDM symbol period, the send antenna TX1 sends a pilot OFDM symbol and at the same time the OFDM symbol sent by the antenna TX2 includes only null carriers, typically modulated by null symbols sn. In the second OFDM symbol period, the send antenna TX2 sends a pilot OFDM symbol and at the same time the OFDM symbol sent by the antenna TX1 includes only null carriers, typically modulated by null symbols sn. This technique reduces the spectral efficiency of all the send antennas compared to a single-antenna frame. It requires a number of OFDM symbols for estimating the channels equal to the number of send antennas and the number of pilot OFDM symbols present in an SISO (Single Input Single Output) frame is consequently multiplied by the number of send antennas.
A second technique, illustrated by FIG. 1b, described in the paper by J. Moon, H. Jin, T. Jeon and S.-K. Li “Channel estimation for MIMO-OFDM Systems employing Spatial Multiplexing”, Vehicular Technology Conference, Vol. 5, September 2004, sends in the same OFDM symbol period a pilot symbol at a particular carrier frequency fp at one send antenna and a null symbol sn at the same frequency at the other send antennas, which avoids interference with the received pilot symbol sp. FIG. 1b illustrates this technique for constructing type 2 frames for two send antennas TX1 and TX2. This technique amounts to using non-contiguous sets of sub-carriers for the training sequences sent at the various antennas. This type of construction leads to a loss of spectral efficiency because of the presence of imposed null symbols sn in an OFDM symbol concomitant in time with an OFDM symbol including pilot symbols sp. After sending a pilot OFDM symbol, the antenna TX1, respectively TX2, can send data symbols sd at the various carrier frequencies.
A third technique, illustrated by FIG. 1c, described in the paper by E. G. Larsson and J. Li, “Preamble Design for Multiple-Antenna OFDM-Based WLANs With Null Subcarriers”, IEEE Signal Processing, Vol. 8, No. 11, 2001, involves constructing training sequences by Alamouti-type space-frequency coding. FIG. 1c illustrates this technique for constructing type 3 frames for two send antennas TX1 and TX2. A major drawback of this kind of technique is that, because of the orthogonal pattern mo, it increases the number of pilot symbols sp in an OFDM symbol compared to a SISO frame and assumes that the channel is constant over a certain number of sub-carriers.
To remove the constraints of previous techniques, some linked to the imposed presence of null symbols, in order to retain the disposition of the pilot symbols of a single-antenna frame between the various send antennas, and in order to use the same set of sub-carriers for all pilot frequencies, a known solution uses the principle whereby a pulse, or more generally a reference sequence, is sent at each send antenna and is shifted in time so that the receiver connected to each receive antenna can isolate in the time domain the impulse responses of the various transmission channels.
This principle is employed in the techniques described in the paper by M.-S. Baek, H.-J. Kook, M.-J. Kim, Y.-H. You and H-S. Song, “Multi-Antenna Scheme for High Capacity Transmission in Digital Audio Broadcasting”, IEEE Transactions on Broadcasting, Vol. 51, No. 4, December 2005 and in the paper by I. Barhumi, G. Leus and M. Moonen, “Optimal Training Design for MIMO OFDM Systems in Mobile Wireless Channels”, IEEE Transactions on Signal Processing, Vol. 51, No. 6, June 2003. In the first of those papers, the set of sub-carriers of an OFDM symbol is dedicated to channel estimation, enabling the receiver to recover the various impulse responses prior to OFDM demodulation. In contrast, in the second paper, multiplexing the payload data symbols and the pilot symbols, where the pilot symbols are distributed over one or more OFDM symbols to form a training sequence, implies that the impulse response recovery operation in the receiver is effected after OFDM demodulation. This operation employs a matrix A constructed from the training sequence and a Fourier matrix with appropriate dimensions. The coefficients of the various impulse responses are estimated by multiplying the demodulated received signal by the pseudo-inverse matrix of the matrix A.
The Baek technique has the advantage over the Teng technique of avoiding the imposed presence of null pilot symbols and thus offers higher spectral efficiency. It has the advantage over the Moon technique that it estimates the channel for all modulated carriers. The Barhumi technique offers greater spectral efficiency than the Teng, Moon and Larsson techniques. Table 1 in Appendix A (see below) compares the relative amount of payload data for a given number of OFDM symbols per frame for the various techniques referred to above and for two send antennas.
However, the Baek and Barhumi techniques offer poor performance if the time-frequency frames to be sent include null carriers at the edges of the spectrum, these edge null carriers typically being used to reduce the spectral occupation of the sent signal that can interfere with adjacent bands. These techniques then lead to edge effects that degrade system performance compared to a system with perfect channel estimation.